Dithiophosphate accelerators have been used as sulfur donors in the vulcanization of diene. As compared with conventional accelerators, they offer advantages such as improved reversion of natural rubber, heat stability of the vulcanizates or rubber produced, and high solubility in rubbers of varying polarity for curing and producing a product without bleed of additive (“non blooming”).
Commercially available dithiophosphate additives are (1) “Accelerator ZIPPAC” from Akrochem. ZIPPAC is a zinc-amine dithiophosphate complex dispersed in a polymer matrix. ZIPPAC, when used in combination with thiazoles and thiurams, gives fast cure rates to ENB type EPDM's. Compared to Akrochem's other thiophosphate accelerator, Accelerator VS, ZIPPAC provides a much faster cure. ZIPPAC cures exhibit high modulus and low compression set, due to a high degree of crosslinking. Vulcanizates are usually non-blooming. Hot air aging properties are usually better than those obtained with other thiophosphate accelerators. ZIPPAC does not form harmful nitrosoamines (Zippac forms only unstable primary amines that rapidly decompose) and is a useful accelerator in low nitrosoamine generating formulas where its high crosslink density and fast cure overcome normal problems associated with safe nitrosoamine cure systems; (2) Royalac 136 from Crompton-Uniroyal Chemical is a zinc thiophosphate (zinc phosphorodithioate); (3) Meramid P a zinc dialkyl dithiophosphate under CAS number 68457-79-4; (4) Deovulc EG 187 from D.O.G. which is described as a synergistic combination of Thiazole- and basic accelerators together with a zinc dialkyldithiophosphate (ZDTP) as the main components; (5) Vocol ZBPD from Flexsys is described as Zinc O,O-Dibutylphosphorodithioate. Elastomer and utilized in NR, EPDM, LATEX rubber and non-blooming accelerator for EPDM cures and also improves reversion resistance in NR; (6) VANAX® 196 from R. T. Vanderbilt Company, which is a dithiophosphate disulfide; (7) Rhenocure AP-6 from Rhine Chemie, which is described as dithiophosphate polysulfides; (8) Rhenocure® TP/S from Rhine Chemie which is described as a Dithiophosphate, sulfur source; (9) Rhenocure® SDT/S from Rhine Chemie which is described as a phosphoryl polysulfide, sulfur source; (10) Rhenocure SDT/G from Rhine Chemie which is also is described as a phosphoryl polysulfide, sulfur source; and (12) Rhenogran® SDT-50 from Rhine Chemie which is described as a polymer bound phosphory polysulfide.
Nitrosoamine reduction in rubbers has become important due to the carcinogenic properties linked to nitrosoamines. Dithiophosphate accelerators have a renewed interest due to lack of nitrogen in the chemical structure, which is pre-requisite of nitrosoamine formation. A common expression for having better environmental properties is “green” and efforts are in way to produce more and more green additives.
One concern over dithiophosphate technology is that although it can not form harmful nitrosoamines, the technology has the ability to be a toxin. Malathion is a widely available dithiophosphate insecticide. It is not only toxic to insects but to a wide variety of animal life (The Ortho Group, “Environmental Hazards: This pesticide is toxic to fish, aquatic invertebrates, and aquatic life stage of amphibians.”). Other references to toxic dithiophosphates are provided in U.S. Pat. No. 2,725,327 Nov. 29, 1955, William R. Diveley; et al., which describes Pestisidal composition containing 2-P-Dioxanethiol S—(O,O-dialkylphophorodithioate) and U.S. Pat. No. 2,828,241 Mar. 25, 1958, Gail H. Birum; et al. which describes O,Odialkyl S-arylmercapto phosphorodithioates compositions and method of destroying insects.
It is generally accepted that polymeric materials offer improved and lower toxicity over their monomeric forms. Bonding an additive to a polymer (carrier) would afford improved environmental safety but dilutes the activity of the molecule by non active polymer (as related to rubber cure). It has been unexpectedly discovered that forming polymeric dithiophosphates via bi- or polyfunctional alcohols (utilization of di- or polyol alcohol within the reaction with phosphorous pentasulfide to form the oligomer or polymeric dithiophosphoric acid) can be oxidized to oligomeric or polymer disulfides and subsequently to polysulfides. Alternatively the dithiophosphoric acid can be reacted with metal hydroxides or oxides. The resulting product will function as an “additive” which can offer high activity, excellent vulcanization and/or stabilization properties to rubbers and offer improved handling and environmental (“green”) consideration due to their polymeric additive form (as compared to the monomeric type additives).